Which factor does NOT affect the rate constant k of a reaction?
A
Temperature
B
Concentration of reactants
C
Nature of reactants
D
Presence of catalyst
Correct Answer:
B. Concentration of reactants
Rate constant k is independent of reactant concentration; it depends on T, nature of reactants, and catalyst
Consider the mechanism: (1) A + B ⇌ C (fast equilibrium), (2) C + D → E + F (slow). Which is the rate-determining step?
A
Step 1
B
Step 2
C
Both steps
D
Cannot be determined
Correct Answer:
B. Step 2
The slowest step in a mechanism is the rate-determining step (RDS), which is step 2
For a zero-order reaction, the integrated rate law is [A] = [A]₀ - kt. If [A]₀ = 0.5 M and k = 0.02 M·s⁻¹, find time when [A] = 0
A
15 s
B
20 s
C
25 s
D
30 s
0 = 0.5 - 0.02t; t = 0.5/0.02 = 25 s
Which of the following statements about collision theory is INCORRECT?
A
Molecules must collide with proper orientation
B
All collisions lead to reaction
C
Collision frequency increases with temperature
D
Only collisions with energy ≥ Eₐ are effective
Correct Answer:
B. All collisions lead to reaction
Not all collisions are effective; only those with proper orientation and sufficient energy lead to reaction
Half-life of a first-order reaction is 30 minutes. What fraction of reactant remains after 90 minutes?
After 90 min = 3 half-lives, remaining = (1/2)³ = 1/8
For the reaction 2NO + O₂ → 2NO₂, the rate law is Rate = k[NO]²[O₂]. What is the overall order of reaction?
A
First order
B
Second order
C
Third order
D
Zero order
Correct Answer:
C. Third order
Overall order = sum of exponents in rate law = 2 + 1 = 3 (third order)
For an elementary reaction: A + A + B → Products, the rate law is:
A
Rate = k[A][B]
B
Rate = k[A]²[B]
C
Rate = k[A][B]²
D
Rate = k[A]³[B]
Correct Answer:
B. Rate = k[A]²[B]
For elementary reactions, the rate law is determined by the stoichiometry: Rate = k[A]²[B], as 2 A molecules and 1 B molecule participate
For the reaction A → B, experimental data shows: [A] vs time is a straight line. This indicates:
A
First-order reaction
B
Zero-order reaction
C
Second-order reaction
D
Third-order reaction
Correct Answer:
B. Zero-order reaction
A straight line in [A] vs time plot indicates [A] = [A]₀ - kt, which is the integrated rate law for zero-order reactions
For a reaction with rate law Rate = k[A]⁰[B]¹, if [B] is increased 4 times while [A] remains constant, the rate increases by:
A
2 times
B
4 times
C
8 times
D
16 times
Correct Answer:
B. 4 times
Since the reaction is first-order in B and zero-order in A, Rate ∝ [B]. Increasing [B] by 4 times increases Rate by 4 times
A catalyst increases the rate of reaction by:
A
Increasing activation energy
B
Providing an alternative reaction pathway with lower activation energy
C
Increasing the frequency factor
D
Changing the equilibrium constant
Correct Answer:
B. Providing an alternative reaction pathway with lower activation energy
A catalyst works by providing an alternative mechanism with lower Ea, thus increasing reaction rate without being consumed
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